1. Field of the Invention
The present invention relates to a nitride semiconductor field effect transistor (FET) and a method of fabricating the same, and more particularly, to a nitride semiconductor FET with an ohmic metal layer and a method of fabricating the same.
2. Description of the Related Art
Semiconductor materials such as Si or GaAS have been widely used for semiconductor device fabrication, but it has been found that they are difficult to apply to high-performance and high-frequency devices. Numerous attempts to develop alternative semiconductor materials have resulted in GaN compound semiconductors. GaN compound semiconductors are group-III nitrides with a wide bandgap energy, high thermal/chemical stability, high electronic mobility, and high electron saturation velocity. Thus, GaN compound semiconductors can be applied to new generation mobile communication systems or satellite communication systems which require high-performance and high-frequency, and to engine control systems requiring high thermal resistance.
However, during fabrication of a conventional nitride semiconductor FET, since the bandgap energy of a GaN layer is different from that of a source/drain ohmic electrode into which current flows, a bandgap offset is caused, and the bandgap offset forms a potential barrier.
Since current cannot smoothly flow into the source/drain ohmic electrode due to the potential barrier, a large voltage drop is caused between the GaN layer and the source/drain ohmic electrode. The large voltage drop increases a driving voltage of the nitride semiconductor FET, which reduces an operating efficiency of the nitride semiconductor FET.
Furthermore, if the driving voltage of the nitride semiconductor FET is increased to operate a high-performance device, a high-temperature exothermic reaction occurs and causes thermal degradation of electrode materials. As a result, the nitride semiconductor FET has a reduced operating efficiency and degraded electrode materials, which leads to degradation of the nitride semiconductor FET.